In DE 197 04 740 A1 a display system of this kind is disclosed for aircraft and motor vehicles. In it a light source is provided which beams polychrome laser light onto an imaging unit designed as an LC microdisplay (Liquid Crystal Device), DMD microdisplay (Digital Micromirror Device) or scanning device. By appropriate control, especially computer control, the imaging unit information is modulated onto the light signal emitted by the light source. The modulated light is steered onto a holographic beam combiner which the user, e.g., the driver of a motor vehicle, views.
The term, holographic beam combiner, is understood to mean the hologram, recorded on an appropriate support, of a fitted, preferably white image screen. The hologram is produced in the hologram plane by superposing on a reference beam the object rays scattered by the appropriately illuminated image screen. Later irradiation of the hologram with a projection beam, which is emitted by a projection unit which is in the same spatial relationship to the hologram as the reference beam source was while it was being photographed, produces a virtual image, perceivable by a suitably positioned observer of the holographically photographed image screen or corresponding partial areas thereof. The observer gains the impression of an image screen which is in the same position relative to the beam combiner as the object image screen was positioned relative to the hologram plane. Thus displays can be created which appear to be positioned “in infinity” which places the driver of a motor vehicle in a position to read the displays on a holographic beam combiner arranged in the area of his dashboard without refocusing his eyes, which had been focused “at infinity” to observe the traffic, to the distance of the dashboard that is usually close to him. The cited disclosure discloses corresponding beam combiners for incident light and rear projections, i.e., for use in transmission and reflection. Another advantage of the known system lies in the limitation of the radiation of light from the holographic beam combiner into an angle in space that can be established by the photographing geometry. A holographic display system for aircraft, which can be used in reflection is disclosed in DE 693 05 713 T2.
It is disadvantageous in the known systems that the holographic beam combiner has to be especially made in regard to its size for every application. For adaptation to special installation conditions, complicated optical methods are necessary for scaling the holographic interferogram. An additional disadvantage is to be seen in the fact that the illumination of the holographic beam combiner with free laser beams constitutes a not inconsiderable safety risk, e.g., for the passengers of a motor vehicle interior swept by laser beams.
It is the object of the present invention, therefore, to create a holographic display system which is simpler to manufacture and more flexible and safer in use.
This object is accomplished by the invention by the fact that in and/or on the vehicle at least one observation mirror is provided by which light radiated by the holographic beam combiner can be directed into the optical perception range of a user.
By the configuration according to the invention the holographic beam combiner itself is no longer used as a viewing element as in the state of the art. This function is assigned instead to the viewing mirror. The holographic beam combiner acts instead as the holographic rear projection screen which is viewed only indirectly through the viewing mirror.
Thus it is possible to arrange the holographic beam combiner in spatial proximity to the projection unit, as provided in a preferred embodiment. Accordingly, the paths of the free light beams, preferably laser beams, can be kept extremely short, so that no endangerment of the viewer or to third parties is created. Instead, the light from the holographic beam combiner spread out in a preferred angle passes only through the space between the projection unit and viewing mirror
Furthermore, the holographic beam combiner can be manufactured in a standard size which can be optimally adapted to the imaging unit. Adaptation to any particular installation conditions is unnecessary.
In a preferred embodiment, the path of light between the holographic beam combiner and the projection unit is optically shielded [“encapsulated”] externally. Thus safety is enhanced, which is important particularly in the comparatively restricted interior of motor vehicles.
Preferably, the light source comprises a plurality of lasers of different output wavelengths. They can be continual-sweep lasers. To modulate the light signal with the desired image information, an appropriate operation of the imaging unit is all that is needed, which is preferably a programmable computer program. It is also quite possible for the light source to comprise a laser with a controllable output wavelength. In such a system a computer program might control both the imaging unit and the light source, in which case the image build-up would itself be subject substantially to the control of the imaging unit and the coloring of the control of the light source. The light source or sources could be, in addition to lasers, also laser diodes, LED's and EELED'S (edge-emitting LED's).
In an especially advantageous embodiment, the viewing mirror has a curved surface. In particular, the surface can be curved such that the viewing mirror offers an observer properly positioned an enlarged image of the holographic image combiner, i.e., acts as an enlarging mirror. Thus the observer can read large and easily recognizable displays despite the minimal space required for the system.
It is especially advantageous if a plurality of viewing mirrors are provided which make the light from a central projection unit perceivable for observers in different positions. For one thing, this can mean that variously positioned observers can view through their viewing mirrors a common holographic beam combiner. On the other hand, it can also be provided such that several observers who themselves are in different positions will be presented by their viewing mirrors different holographic beam combiners illuminated by a central projection unit. Finally, it can also be arranged that images of different holographic beam combiners will be presented to an observer through several differently positioned viewing mirrors. Other obvious combinations are likewise possible.
To avoid a plurality of holographic beam combiners it can also be advantageous, in the scope of the holographic photography, i.e., the production of the homographic beam combiner, to superimpose holograms taken from different angles. In the completed system, this leads to the emission of the spread light from the holographic beam combiner at several preferred angles in which corresponding viewing mirrors can be arranged. If holograms which were photographed in different directions and/or with different wavelengths are superposed in such a photograph, a display system can be achieved with appropriate control of the imaging unit and appropriate design of the light source, in which different images of it can be presented to central holographic beam combiners through variously positioned viewing mirrors.
Especially preferred is an embodiment in which at least one viewing mirror is arranged in the area of the dashboard of a motor vehicle. Thus the customary arrangement of the display units in the motor vehicle can be retained, while nevertheless avoiding any permanent installation of specific display units such as tachometers, rpm displays etc. Instead, it becomes possible through appropriate programming of the display unit to create specific user and/or situation displays.
In an additional advantageous embodiment it is provided that at least one viewing mirror is arranged in the area of a side window or of the windshield of a motor vehicle. This offers the advantage that the driver can read the displays without entirely removing his eyes from the traffic. Such a viewing mirror is preferably not made reflective in the entire visible spectrum. It is especially advantageous to use for this purpose a mirror which is reflective only in the range of wavelengths produced by the light source and is otherwise transparent. In this way the view outward through the window in question is not impaired or impaired only negligibly.
It can furthermore be provided that at least one viewing mirror is arranged in the area of the back of the driver and/or passenger seat of a motor vehicle. This enables passengers on the back seats to be entertained with videos, TV or computer games during the drive. Of course, for this purpose the control of the imaging unit must be appropriately designed.
The imaging unit can be designed in various ways known to the expert. Appropriate for this purpose are microelectromechanical systems (MEM=microelectromechanical system). Among these are DMD microdisplays (DMD=digital micromirror device), biaxially deflectable micromirrors and GLV (grating light valve). Also suitable are designs such as a LCOS microdisplay (liquid crystal on silicon), a scanning system involving two galvanometer mirrors, a combination of galvanometer irrors and MRS (mechanical resonant scanner) or two MRS.
Such elements as well as the corresponding controls are available commercially in hardware and software and therefore can be integrated at reasonable cost. They can be operated together with laser sources or other light sources. The imaging unit in that case can be controlled by a programmed computer.
Additional details of the invention will be found in the following detailed description and the appended drawings in which preferred embodiments of the invention are represented by way of example.